I can confirm dave67’s exact results. (was a 20.04 fresh install. lsb_release -a gives 20.04.02. )
either
$ pcmanfm-qt --desktop-pref=anything
or
Preferences > LXQt settings > Desktop > Advanced
Show menus provided by window managers when desktop is clicked
When enabled, pcmanfm-qt defaults to the openbox menu in
usr/share/lubuntu/openbox/menu.xml
dave67 is right, it is a ‘throw back’ to the ‘good ol days’.
So should Lubuntu hide this option ?
Or make use of it by replacing the menu entries ?
e.g
I replaced lxsession-default calculator with kcalc and it works.
(btw = somebody was asking in another topic about putting in user entries for right-click menu, I actually think he meant this menu, and not the file-manager menu). So this is a solution, if he meant that.)